The gripping force F of a tire can be given by the product of the frictional coefficient .mu. between the tire and the road surface and the vertical contact load W acting on the tire contact surface (F=.mu.W). In other words, the tire gripping force which plays an important role in the handling of the vehicle is proportional to the magnitude of the tire contact load for a given road condition.
It is well known that the handling of a vehicle is severely impaired when a road wheel thereof is locked up when braking. In particular, on a road surface such as a frozen road surface and a gravel road surface which could significantly reduce the gripping force of the tire, it is important to apply as large a braking force to the wheel as possible, but, without locking the wheel. The antilock brake system (ABS) makes use of this principle, and is widely used in road vehicles now on the market. According to this system, by noting the relationship between the tire slip ratio (.lambda.=(V.sub.v -V.sub.w)/V.sub.v : the ratio of the difference between the vehicle speed V.sub.v and the tire speed V.sub.w to the vehicle speed V.sub.v) and the tire grip force F, the slip ratio is kept within a limit typically by intermittently releasing the brake so as to avoid excessive slipping and to provide a high gripping force at all times. However, the ABS system is not able to change the gripping force of the tire. In other words, the capability of the ABS system to reduce the braking distance of the vehicle is limited by the given road gripping force. It is therefore desirable if the gripping force itself can be increased in view of further reducing the braking distance.
The road gripping force of a tire is important also when accelerating a vehicle. When a vehicle is excessively accelerated for a given road condition, the tires start slipping, and not only a desired acceleration is prevented from being achieved, but also the lateral stability of the vehicle may be lost. By noting this problem, it has been proposed to control the traction force of each driven wheel so that the slip ratio of the wheel may be kept within a limit, and a maximum available traction may be obtained at all times. The traction control system is designed to carry out such a control action. However, the conventional traction control system was not able to increase the magnitude of the available traction, and simply reduces the torque transmitted to the wheels so as to prevent the slip ratio from exceeding a prescribed limit.
When a linear actuator interposed between a wheel and a vehicle body is either extended or retracted at a certain acceleration, a corresponding inertia force is produced in the sprung mass and the unsprung mass. The reaction of such an inertia force may be used to increase the contact load of the road wheel or the gripping force of the tire. Therefore, when this concept is applied to a brake control system or a traction control system, it is possible to reduce the braking distance or to increase the magnitude of the available traction. In particular, the inventors have recognized that an optimum result can be achieved if the contact load of the wheel is increased when the slip ratio of the wheel is about to exceed a threshold level beyond which the tire grip force starts diminishing.